...And we shall mention some things related to repentance (towbah) that are very necessary (to grasp), and it is not befitting that someone be ignorant of them.

From them: Immediate repentance is obligatory after committing a sin. It is not permissible to delay it. Whenever someone delays it, he has committed (another) sin by delaying it. In this case, when he repents from the sin (itself), another act of repentance remains a duty upon him - that he must repent for delaying his repentance! This is something that rarely comes to mind when someone repents. Instead, he may assume that since he has repented from the sin, that there is nothing else left to do, while there remains upon him the duty to repent from delaying his repentance...

NOTE: The quote from Ibn al-Qayyim is actually from his book Madaarij as-Saalikeen, the section on Towbah. The book called "at-Towbah" attributed to Ibn al-Qayyim (referred to above) was extracted from Madaarij as-Saalikeen, and it is not a seperate book with that name. So the quote should actually be referenced to Madaarij as-Saalikeen, not "Kitaab at-Towbah".